You can't open an on-premises or legacy dedicated mailbox by using OWA in Office 365 dedicated/ITAR vNext

Symptoms

You can't open an on-premises or legacy dedicated mailbox by using Microsoft Outlook Web Access (OWA) in Microsoft Office 365 dedicated/ITAR vNext.

Resolution

To resolve this issue, try to access an on-premises or legacy dedicated mailbox from OWA to which you have Full Access permissions. To do this, follow these steps:

Sign in to the cloud mailbox by using OWA.

Select Open Another Mailbox.

Open the on-premises or legacy dedicated mailbox. You'll be redirected to on-premises or legacy dedicated OWA.

Enter your credentials.

Instead of being routed to the on-premises or legacy dedicated mailbox, you're redirected to the cloud OWA URL. When you enter your credentials again, you will sign in to your own mailbox.

Workaround

To work around this issue, sign in to the on-premises or legacy dedicated mailbox by using the on-premises OWA URL. Then, specify the on-premises SMTP address. For example, the URL resembles the following:

https://mail.contoso.com/owa/sharedmbx@contoso.com

When you're prompted for credentials, you should enter your account credentials.